Accurate measurement of the angular motions of spinning bodies with on-board sensors has been recognized as a significant contribution to the research and development of experimental projectiles and rockets and to the diagnosis of existing munitions systems. Sun sensor arrays, such as described in U.S. Pat. No. 5,909,275 have been developed to measure the attitude of a spinning body utilizing the sun as a parallel light source. These devices are restricted to bright sunshine conditions and require an external modification to the body to physically accommodate the sensors.
Devices responsive to the earth""s magnetic field have long been used for estimating heading. Traditional compasses indicate only the direction of the horizontal component of the earth""s field, whereas it is often desired to have knowledge of the angle of the body with respect to the local magnetic field. The orientation of the field with respect to a three-dimensional, earth-fixed coordinate system changes both with location and with time. It is important to note that determination of orientation with respect to the earth""s magnetic field in three-dimensional space does not uniquely specify orientation with respect to the earth (or other possible navigation frames); another datum is required, e.g., a vertical reference.
Existing magnetic sensing devices require some or all of the following for relative heading measurements: moving parts, three dimensions of sensor measurements, knowledge of the magnetic field components, knowledge of the strength of the magnetic field, gain calibration of the sensor(s), and sensor temperature compensation.
Additionally, all other known systems giving orientations with respect to a magnetic field make those determinations from one or more of four basic measurement types: 1) field strength along a sensor axis, 2) relative field strength along multiple sensor axes, 3) rate of change of field strength along a sensor axis, 4) relative rates of change along multiple sensor axes. In every case, the measurements are premised on some evaluation of a component of the magnetic field along a sensor axis.
For example, U.S. Pat. No. 4,767,988 discloses a device with three axes of magnetometers in two distinct planes that must be rotated about at least two of these axes to determine orientation within a magnetic field. This requires that the sensor move with respect to the platform on which it is mounted. This also requires that the gain of each of the three sensor axes be known as the sensor processing methodology depends upon accurate measurement of changes in field strength with changes in sensor orientation. The data processing is computationally demanding, requiring many matrix operations and averaging of many computed quantities.
The system of the present invention makes the same measurement of magnetic heading but has no moving parts, has a single plane of sensor measurements, and has no gain calibration requirement of its sensors. Further, the only computations required are two scalar subtractions, a scalar division, and a look-up (and possible interpolation). The simplicity of this methodology makes feasible real-time, on-board processing.
It is the primary object of the present invention to provide a system and a simple, robust methodology wherein one or more magnetometers sensitive to the strength of a magnetic field can be used to determine the magnetic attitude and magnetic roll position of a rotating body with respect to that field.
The system can be used for the estimation of the in-flight attitude, that is, the angular orientation, to an accuracy of 0.1 degrees of spinning bodies with respect to the earth""s magnetic field. This use relates to a configuration incorporating one or more magnetometers and a method for determining the orientation of the apparatus relative to the earth""s magnetic field. This determination is made on the basis of the magnetometer(s) phase information during a roll cycle and is amplitude independent. Manufacturing tolerances on scale factor and/or gain variations, which are the bane of many inertial measurement units (IMUs), have no effect on system performance of the present invention.
The system provides a unique all-weather, day/night magnetic angular measurement capability for spinning bodies that does not currently exist. In addition, this system may supplement or include other measurement techniques in a hybrid configuration as to provide further IMU-type data and could replace some existing optical measurement techniques wherein an exterior surface needs to be compromised. Potential applications for the system include, but are not limited to, orientation and attitude of any rotating body including navigation aids, rockets, projectiles, satellites, and deep-space exploration vehicles. The measurement capability can be integrated into the guidance and control capabilities of airborne spinning bodies without regard to their exterior geometry.
To obtain an indication of the angular orientation of a rotating body, relative to a magnetic field, such as the earth""s magnetic field, a sensor array having at least one magnetometer is placed within the body prior to launch. During flight, the magnetometer will provide an output signal which varies between positive and negative values and which has periodic zero crossings during flight and as the body rotates.
A look-up table is generated, preferably prior to flight, of a roll angle discriminant, based upon zero crossings, versus angular orientation. A time discriminant is continuously obtained, based upon the zero crossings during actual flight of the spinning body through the earth""s magnetic field. During, or after the flight, the look-up table is accessed to ascertain the closest match between a time discriminant and a roll angle discriminant to obtain a flight history of the values for the angular orientation of the body with respect to the magnetic field.